In high-speed optical communication, DQPSK is considered as the most potential modulation format. In the industry, researches on the DQPSK modulation format are competitively performed, and an orthogonal polarized frequency division multiplexed-return to zero-differential quadrature phase shift keying (OPFDM-RZ-DQPSK) modulation format is proposed, which has great tolerance on polarization mode dispersion (PMD), chromatic dispersion (CD), and non-linearity.
FIG. 1 shows a working principle of the OPFDM-RZ-DQPSK modulation format. A frequency of a driving signal of a Mahe-Zender modulator (MZM) is f0, and by setting a bias point and an amplitude of the driving signal of the MZM, a light having a frequency of f emitted from a laser diode (LD) is modulated by the MZM, which outputs two optical sub-carriers SC1 and SC2. Frequencies of the two sub-carriers are respectively fSC1=f+f0 and fSC2=f−f0. Then, the sub-carriers SC1 and SC2 are separated in the frequency domain by a demultiplexer (Demux). An RZ-DQPSK signal is respectively modulated on the sub-carrier SC1 and the sub-carrier SC2, the SC1 RZ-DQPSK signal is kept in an X-polarization state, and the SC2 RZ-DQPSK signal is kept in a Y-polarization state. Finally, the SC1 RZ-DQPSK signal and the SC2 RZ-DQPSK signal are sent to a polarization beam combiner (PBC) for processing to obtain an OPFDM-RZ-DQPSK signal.
In the conventional art, the OPFDM-RZ-DQPSK signal is usually received through delay interference. FIG. 2 shows a device for receiving the OPFDM-RZ-DQPSK signal in the conventional art. Referring to FIG. 2, an OPFDM-RZ-DQPSK signal is received and de-multiplexed by a Demux into an SC1 signal and an SC2 signal. The SC1 signal is split by a coupler into two signals having the same power, and delay demodulation is performed on the two signals respectively by using a Mahe-Zender interferometer (MZI). The SC2 signal is also split by a coupler into two signals having the same power, and delay demodulation is performed on the two signals respectively by using an MZI. Finally, balance detection is performed on each MZI output signal with a detector, so as to obtain an original signal.
During the process of implementing the present invention, the inventor found that in the receiving method of the conventional art, the two sub-carriers are separated in the frequency domain by using the Demux, but it is difficult for the Demux to completely eliminate crosstalk caused by the interaction of the two sub-carriers in the frequency domain, resulting in error recovered data.